The invention relates generally to imaging systems, and particularly relates to imaging optics for use in relatively high power imaging systems.
Conventional imaging systems typically include a modulated illumination system for generating a modulated field of illumination, and an optical assembly for applying the modulated field of illumination to an imaging surface. Such illumination systems may provide a line of laser illumination so that a line of picture elements (or pixels) may be imaged at a time for efficiency in imaging. The field of illumination may be modulated by selectively controlling the illumination system (e.g., as disclosed in U.S. Pat. No. 4,804,975), or by using a light modulator for selectively modulating the field of illumination. Illumination systems that modulate the illumination field generally require that relatively high currents be switched on and off at fairly high speeds. The use of light modulators permits the illumination system to provide a relatively uniform field of illumination by permitting the laser emitters to exhibit relatively uniform power consumption and be maintained at a relatively uniform temperature.
Optical assemblies for applying the modulated field of illumination to an imaging surface generally include imaging optics that focus the modulated field of illumination on an imaging surface, such as a drum. Certain such optical assemblies further include automatic focusing apparatus. For example, U.S. Pat. No. 5,212,500 discloses an imaging system for a color proofing apparatus in which a secondary light source is generated that is projectable through the optical lens assembly onto the writing element. The secondary light source is then reflected off of the writing element and received at a photodetector. A movable lens element is then adjusted for automatic focusing of the optical assembly responsive to the signal generated by the photodetector. Because the system of the ""500 patent is a color proofing apparatus involving dye sublimation, the power level of the primary light source for writing is relatively low, and images may be printed responsive to the reception of a small amount of light at the writing surface. Consequently, the secondary light source must be of a wavelength that is different than the wavelength of the primary light source to prevent the secondary light source from causing any images to be formed on the writing surface. Also, some undesired light (or noise), may be received by the photodetector. This is because any reflections of the secondary light off of the optical lenses in the optical assembly will be reflected back along the path of the secondary light toward the photodetector. Because the secondary light source produces a relatively small amount of light, any such reflections may adversely affect the automatic focusing assembly, particularly if the primary light source is relatively high power. Also, photodetectors that are responsive to power only, provide an indication of corrective distance but not corrective direction.
It is an object of the invention to provide an optical assembly that efficiently and accurately provides automatic focusing.
It is another object of the invention to provide an automatic focusing optical assembly for use in a high power imaging system.
It is another object of the invention to provide an automatic focusing optical assembly in a thermal imaging system for imaging thermally sensitive media.
It is another object of the invention to provide an automatic focusing optical assembly using a focusing sensor that is not responsive to power.
It is another object of the invention to provide an automatic focusing optical assembly with minimal undesired light (or noise) appearing at the focusing sensor.
It is another object of the invention to provide an automatic focusing optical assembly with a focusing sensor that provides an indication of the corrective direction as well as the corrective distance.
The invention provides an imaging system and method including a first illumination source for producing a first illumination field, a modulation system for modulating the first illumination field, and an optical imaging system for directing the modulated illumination field to an imaging surface. The optical imaging system includes at least one optical lens for directing the modulated illumination field toward the imaging surface along the optical axis of said optical lens. The at least one optical lens includes a first surface facing the modulation system and a second surface facing the imaging surface. The optical imaging system also includes a second illumination source for producing a second illumination field. The second illumination field has a frequency that is different than the frequency of the first illumination field. The optical imaging system also includes an optical assembly for directing the second illumination field toward the imaging surface along an optical path that is different than the first optical path such that the second illumination field contacts the first surface of the at least one optical lens from a direction other than that which is normal to the first surface of the at least one optical lens. The optical imaging system also includes a sensor assembly for receiving the second illumination field after it is reflected off of a portion of the imaging surface, and for producing a sensor output signal. The optical imaging system also includes a control assembly for adjusting the position of the at least one optical lens responsive to the sensor output signal.